The announcement by Honda can be found Here

Here is part of the announcement:
Katsushi Inoue, Chief Officer for European Regional Operations, Honda Motor Co., Ltd., and President, Honda Motor Europe said; "In light of the unprecedented changes that are affecting our industry, it is vital that we accelerate our electrification strategy and restructure our global operations accordingly. As a result, we have had to take this difficult decision to consult our workforce on how we might prepare our manufacturing network for the future. This has not been taken lightly and we deeply regret how unsettling today's announcement will be for our people."
. . .
Honda's European HQ will continue to be located in the UK. It will be focused on serving the needs of our European customers.

This gives a good reason for the restructuring (and thus the closure of the plant) and does not mention Brexit.
This is not selective quoting. The rest of the page doesn't mention Brexit either.